A helicopter commonly has a main lift and propulsion rotor that is provided with a plurality of blades.
The blades of the main rotor describe a very flat cone, referred to as the “rotor cone” by the person skilled in the art, with the plane of rotation thereof being perpendicular to the general lift generated by the main rotor. This general lift of the main rotor may then be resolved into a vertical lift force and a horizontal force that drives the helicopter in translation.
Consequently, the main rotor provides a helicopter both with lift and with propulsion.
Furthermore, by controlling the shape and the angle of inclination of the rotor cone relative to the frame of reference of the helicopter, a pilot can control the helicopter with precision.
In order to act on the rotor cone, the blades are caused to flap so as to modify their angles of incidence relative to the drive plane of the rotor, said drive plane being perpendicular to the mast of the rotor.
As a result, the helicopter is provided with specific means serving to vary the pitch of each blade, and consequently to vary the aerodynamic angle of incidence of each blade relative to the incident stream of air through which the blade is passing.
By causing the pitch of a blade to vary, the lift it generates is modified, thereby causing the blade to flap.
In order to control the general lift of the rotor, both in magnitude and in direction, the helicopter pilot thus acts generally on the value of the pitch angle of each blade by causing the blade to turn about its longitudinal pitch axis.
Thus, when the pilot causes the pitch of the blades to vary collectively, i.e. causes identical variation in the pitch of all of the blades, that causes the magnitude of the general lift of the main rotor to be varied so as to control the altitude and the speed of the helicopter.
In contrast, collective pitch variation has no effect on the direction of said general lift.
In order to modify the direction of the general lift generated by the rotor, it is appropriate to cause the rotor cone to be inclined by causing pitch to vary cyclically as opposed to collectively. Under such circumstances, the pitch of a blade varies as a function of its azimuth direction, and during one complete revolution it passes from a maximum value to a minimum value, which values are obtained in diametrically opposite azimuth directions.
Causing the pitch of the blade to vary cyclically gives rise to cyclical variation in the lift of the blade and thus varies the angle of inclination of the rotor cone. By controlling cyclic pitch variation of the blades, the pilot controls the attitude of the aircraft and its movement in translation.
U.S. Pat. No. 2,534,353 discloses a first device for controlling the pitch of the blades of a helicopter.
According to that document, a helicopter rotor is fitted with two blades each secured to a sleeve that is attached to a hub.
The pilot controls the collective pitch of the blades by means of a lever acting on a rod housed inside the rotor mast. The rod delivers its movement to first and second rods attached to the hub. By moving the collective pitch control lever, the pilot causes said rod to move in translation, thereby causing the hub, and consequently the blades, to turn about a pitch variation axis.
The hub is also secured to first and second lift elements arranged in the plane of the blades via first and second connection shafts rigidly connected together, the longitudinal axis of the lift elements being perpendicular to the longitudinal axes of the blades. These lift elements are referred to as “paddles” by the person skilled in the art.
Each connection shaft is also connected to a control plate known as a swashplate via scissors linkage. More precisely, the swashplate comprising a rotary plate and a non-rotary plate, the scissors linkage are secured to the rotary plate of the swashplate.
In addition, the non-rotary plate has a stick that the pilot can grasp.
In order to control the cyclic pitch of the blades, the pilot moves the stick to incline the non-rotary plate, and consequently to incline the rotary plate. The inclination of the rotary plate is then transferred to the first and second connection shafts via the scissors linkage, thereby enabling the pitch of the lift elements to be modified.
The lift generated by the lift elements thus varies, thereby causing them to flap and consequently causing the hub to tilt.
As a result of the hub tilting, the two blades have their own pitch modified.
That first device is relatively simple but it requires the presence of a swashplate that is penalizing both aerodynamically and in terms of weight.
Furthermore, it requires the presence of two subassemblies, respectively for controlling collective pitch variation and for controlling cyclic pitch variation of the blades, thereby leading to large weight and increasing the risk of breakdown.
Finally, the lift elements are supposed to cause the pitch of the blades to vary cyclically. Nevertheless, that does not really happen, strictly speaking, insofar as both lift elements act together on the hub and thus on both blades simultaneously and in identical manner.
Furthermore, it is found that the forces that the pilot needs to deliver in order to incline the lift elements, when acting on the stick of the non-rotary plate, can sometimes be extremely large.
To remedy this particular drawback, a second device is known from U.S. Pat. No. 2,818,123.
According to that document, each lift element is fitted with a tab. The scissors linkages arranged on the rotary plate do not entrain pitch variation of the lift elements, but may cause the tabs to be inclined relative to said lift elements.
By varying the inclination of a tab, the lift of the assembly comprising the lift element and the tab is modified, and consequently the pitch of the blades is modified.
The force the pilot needs to exert in order to vary the pitch of the blades is thus less than in the first device insofar as the lift surface area of the tab is small compared with the lift surface area of the lift elements.
Nevertheless, the above-mentioned drawbacks remain. Furthermore, those first and second devices are, a priori, not applicable to a helicopter having more than two blades insofar as the hub can only turn about a single axis during cyclic pitch variation.
Document DE 2 409 227 discloses another device provided with two lift elements secured to the ends of a single beam passing through the head of the rotor.
Moreover, the state of the art includes a third device used on the modern helicopter.
The collective and cyclic pitch controls of the pilot are connected to three servo-controls via rods and mixers or indeed electrical controls that are secured to the non-rotary plate of a swashplate.
The swashplate is also mechanically linked to each blade by a pitch control rod.
When the pilot seeks to modify the collective pitch of the blades, action is taken on a control that causes the three servo-controls to raise or lower the swashplate assembly, i.e. both the non-rotary plate and the rotary plate.
The pitch control rods are then all moved through the same distance, which implies that the pitch of all the blades varies through the same angle.
In contrast, in order to vary the cyclic pitch of the blades so as to steer the helicopter in a given direction, the pilot causes only one of the servo-controls to move, for example.
The swashplate does not move vertically but instead tilts relative to the mast of the rotor. Each pitch control rod then moves in a direction and through a distance that are specific thereto and the same applies to the pitch of the associated blade.
Pitch control is to some extent individualized, unlike the first and second devices, since each blade is controlled by its own pitch control rod.
That third device is very effective, which explains why it has become widespread. Nevertheless, the forces that need to be applied to control the blades are large, particularly on heavy helicopters, so the servo-controls and the swashplate present weights and dimensions that are large, which is penalizing.
Furthermore, their presence tends to create aerodynamic disturbances.